The present invention relates to a technology for converting color image data, and in detail it pertains to a technology for converting color image data that can be reproduced without any sense of incongruity when reproducing color image data, which have been prepared for a first imaging device, by a second imaging device.
Although there are variable types of apparatuses such as a color display, a color printer, etc., as an imaging device capable of reproducing color images, these imaging devices are different from each other in view of the basic principle and detailed rules for reproducing colors, wherein the ranges of colors (referred as gamuts or color gamuts) which can be reproduced are different from each other in respective apparatuses. For example, a color display reproduces colors based on the principle of a so-called additive color mixture, a color printer reproduces colors based on the principle of a subtractive color mixture. To correspond to differences in such principles, a difference arises in the range of colors that can be reproduced. In addition, even in cases of color displays or color printers, it is normal that these have different ranges of colors in respective apparatuses since, if the types thereof differ, the specifications thereof and/or components are accordingly different from each other.
Thus, since the gamuts of imaging devices differ from each other among the types thereof, it is necessary to execute data conversion when reproducing, by a different imaging device, color image data prepared by a specified imaging device or color image data prepared under the assumption that the data are reproduced by a specified apparatus. For example, where color image data displayed on a color display are supplied to a color printer and the image is printed, it is necessary that, with respect to a gamut which cannot be reproduced by a color printer although the same can be reproduced by the color display, the colors in the gamut are changed to colors which can be reproduced by the color printer and are reproduced. Further, in order to print favorable color images by making the gamut of the color printer as wide as possible and utilizing the same, data conversion is required so that a part of the gamut of the color display is extended to the gamut of the color printer. Such data conversion may be called “mapping.”
Such data conversion (mapping) is executed with reference to a matching relationship of color image data with the matching relationship thereof stored between different imaging devices. For example, where a color image displayed in a color display is printed by a color printer, the matching relationship between the image data displayed in the color display and the image data supplied to the color printer are predetermined, and after color image data used to be displayed on the display are converted in compliance with such a matching relationship, the color image data are supplied to the color printer, thereby printing the color image. And, in order to obtain a favorable color image, it is important to predetermine an adequate matching relationship so that, while absorbing differences in gamuts between different imaging devices, the gamut of an imaging device at the reproducing side can be used as widely as possible. Various methods have been proposed as a method for prescribing such a matching relationship (For example, in Japanese Patent Publication No. 2002-359748A).
However, it is not necessarily easy that a matching relationship is established so that, while absorbing differences in gamuts between different imaging devices, the gamut of an imaging device at the reproducing side is used as widely as possible. In fact, such a problem exists that there are many cases where it is difficult to establish an adequate matching relationship. That is, it is common that the gamut deviates between different imaging devices, and there are many areas in which the gamuts overlap each other. In such areas, the same colors can be reproduced. And, in view of securing the same color appearance not depending on a difference in displaying imaging devices, it is highly recommended that, in such areas where the gamuts overlap, the color image data be converted (mapped) so that the original colors can be retained as accurately as possible. That is, data conversion by which areas that cannot be reproduced due to deviation in the gamuts are compressed within a specified gamut, and to the contrary, areas that are not utilized are extended in the range of the gamut is requested to be executed in only a narrow area where the gamuts overlap each other. As a result, another problem arises, by which continuity of the colors becomes unnatural and the image brings about a sense of incongruity.